1. Field of the Invention
The present invention relates to an optical amplifier for repeating optical signals in an optical fiber system.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing the configuration of a conventional optical fiber repeating and transmitting system, as disclosed, for example, in the Patent Application Laid-Open No. 5-268167. FIG. 2 is a schematic diagram showing the detailed configuration of each of the optical amplifiers therein. In FIG. 1, reference numerals 1a to 1c denote a plurality of optical amplifiers provided in series, numerals 2a to 2d denote power feeding lines, 3a to 3d and 4a to 4d all denote optical fibers, 5 denotes a constant-current power supply for providing a constant current to the power feeding line 2a, and numerals 6a and 6b denote optical transmission terminal devices, respectively provided at the transmission end and the reception end of the transmission system.
As shown above, in the transmission lines 3a to 3d and 4a to 4d connecting the optical transmission terminal devices 6a and 6b, a plurality of optical amplifiers 1a to 1c are provided in series, and the power feeding to each of these optical amplifiers 1a to 1c is carried out normally by a constant-current power feeding method, in which a constant direct current is fed by a constant-current power supply provided in the above transmission end or the reception end of the transmission system by way of the power feeding lines 2a to 2d. 
In FIG. 2, numeral 1a denotes an optical amplifier, 2a and 2b denote power feeding lines, 3a, 3b, 4a and 4b denote optical fibers, numerals 11 and 12 denote optical repeater circuits for amplifying and repeating optical signals, 13 and 14 denote bypass circuits, which are connected respectively to the optical repeater circuits 11 and 12 in parallel for controlling the amount of current for driving the optical repeater circuits 11 and 12 (hereinafter referred to just as xe2x80x9cdrive currentxe2x80x9d), numeral 15 denotes a constant-current power supply connected to the optical repeater circuits 12 and the bypass circuit 14 in series, and numeral 16 denotes a constant-voltage diode connected in parallel to these repeater circuits 11 and 12, the bypass circuits 13 and 14, and the constant-current power supply 15.
Next, the operation of the conventional optical amplifier is now explained as follows.
The up-link optical signal input through the optical fiber 3a is amplified and repeated at the optical repeater circuit 12 and is output to the optical fiber 3b, whereas the down-link optical signal input through the optical fiber 4b is amplified and repeated at the optical repeater circuit 11 and is output to the optical fiber 4a. 
On the other hand, the-optical repeater circuit 11 and the optical repeater circuit 12 are connected to each other in series, and in each of the optical repeater circuits 11 and 12, the bypass circuits 13 and 14 for allowing all or one part of the fed current to pass through are connected in parallel respectively. Due to this, the current input through the power feeding line 2a is branched to the optical repeater circuit 11 and the bypass circuit 13, and also branched to the optical repeater circuit 12 and the bypass circuit 14.
To the optical repeater circuit 11 and the bypass circuit 14, the constant-current power supply 15 is further connected in series, so as to limit the maximum current flowing into the optical repeater circuits 11 and 12. To each of these optical repeater circuits 11 and 12, the bypass circuits 13 and 14 and also the constant-current power supply 15, the constant-voltage diode 16 is connected in parallel. This constant-voltage diode 16 bypasses the excess current when an excessive amount of current is flown into the power feeding line 2a, thereby to implement a stable operation thereof.
As the conventional optical amplifier is constructed as explained above, the operating state of the optical repeater circuits 11 and 12 is controlled to keep constant even when the current fed to the optical amplifier 1a is fluctuated, and due to this, the operating state of these optical repeater circuits 11 and 12 cannot be changed from outside. Subsequently, there has been such a problem that when a malfunction occurs to one of the optical amplifiers due to the long-term use of a transmission system, the output level of other normal state optical amplifiers cannot be changed and so on.
The present invention has been proposed to solve the problems aforementioned, and it is an object of the present invention to provide an optical amplifier which is capable of setting the output level of the optical repeater circuits therein in accordance with the amount of current fed to the optical amplifier.
In order to achieve the above objects, the optical amplifier according to a first aspect of the present invention is constructed such that it comprises: a bypass circuit for controlling the amount of the drive current of the optical repeater circuit on the basis of a setting signal, a voltage limitter circuit that is connected to the optical repeater means in parallel and limits the voltage applied to the optical repeater means, and a current detection means, which is connected in series to a parallel circuit composed of the optical repeater means and the voltage limitter circuit, detects the current amount fed to the parallel circuit, and generates a setting signal to the bypass circuit in accordance with the amount of the fed current thus detected.
Due to this construction, the current detection means generates a setting signal in accordance with the detected amount of the fed current, while the bypass circuit controls the amount of the drive current of the optical repeater circuit on the basis of the thus generated setting signal, so that such an effect that the output level of the optical repeater circuit can be controlled in accordance with the amount of the current fed to the device can be obtained.
Further, due to this construction, since not only the amount of current flown into the optical repeater, but that flown into the voltage limitter circuit can also be detected, total amount of the fed current can be accurately counted, so that a setting signal in accordance with the amount of the fed current can be generated efficiently.
Still further, due to this construction, an excess amount of current can be bypassed, so that a stable operation can be implemented.
The optical amplifier according to another aspect of the present invention further comprises a current limitter circuit, which is connected in series to the optical repeater means for limiting the current flowing into the optical repeater means.
Due to this construction, the maximum current flown into the optical repeater circuit can be suppressed, so that an unstable operation can be prevented.
The optical amplifier according to further aspect of the present invention is constructed such that current detection means further comprises a current detecting section for detecting the fed current, and a filtering section for smoothing a detection signal detected by the current detecting section.
Due to this construction, a fluctuation of the detection signal of the fed current can be smoothed, so that such an effect is obtained that a setting signal can be generated efficiently in accordance with the amount of the fed current.
The optical amplifier according to further aspect of the present invention is constructed such that the current detection means further comprises a current detecting section for detecting the fed current, an impedance converter for converting a detection signal detected by the current detecting section to a voltage signal of low impedance, and a current generating section for generating a current amount setting signal to the bypass circuit in accordance with the voltage thus converted by the impedance converter.
Due to this construction, the current generating section for generating a current amount setting signal to the bypass circuit can be implemented by a relatively simple structure in which only one impedance converter is provided.
The optical amplifier according to the present invention is constructed such that current detection means further comprises a current detecting section for detecting the fed current, an impedance converter for converting a detection signal detected by the current detecting section to a voltage signal of low impedance, and a current generating section for generating a current amount setting signal to the bypass circuit in accordance with the voltage thus converted by the impedance converter.
Due to this construction, the current generating section for generating a current amount setting signal to the bypass circuit can be implemented by a relatively simple structure in which only one impedance converter is provided.
The optical amplifier according to the present invention is constructed such that the current detection means further comprises a setting signal limiting means for making the bypass circuit generate a certain level of setting signal in the case where the amount of the detected fed current is less than a predetermined value.
Due to this construction, a certain level of setting signal is made generated in the bypass circuit in the case where the amount of the detected fed current is less than a predetermined value, so that the bypass circuit can be controlled efficiently.